2.2 Thermodynamics of electrode reactions 19
Figure 2.2: Figures showing the correlation between binding energy and rate of
reaction for Hydrogen Evolution. In the left gure from 48 predicted exchange current
density for hydrogen evolution is plotted as function of the heat of formation for
hydrogen adsorption. In the gure to the right from 49, experimental values of
exchange current densities for hydrogen evolution on various metals are plotted as
function of calculated hydrogen binding energies.
should be maximised when the heat of formation for hydrogen on a catalyst
surface is zero. To the right in gure 2.2 a much later work by Nørskov and
co-workers shows that the hydrogen adsorption energy is a reasonably good
descriptor despite the fact that some of the surfaces in the study oxidise under
reaction conditions 49. It also turns out that platinum is the best catalyst.
Plots of catalyst activity as a function of one or more descriptors are also called
a volcano plots. In the eld of catalysis volcano shaped activity plots are used
extensively and identifying a suitable descriptor for a reaction can be seen as
the rst step towards solid understanding and design principles.
2.2 Thermodynamics of electrode reactions
In electrocatalysis, the primary energy input is the potential dierence over two
electrodes, U. Electrochemical reactions include the transfer of electrons between
the electrode and reactants and the rate of this transfer can be tuned
with U. The electrodes are immersed in an electrolyte which provides ionic
conductivity, due to a high concentration of protons or hydroxide ions. Furthermore,
electrochemical reactions are reduction-oxidation, redox, reactions.
They can always be divided into two half reactions, each proceeding at their
respective electrode. The electrode at which oxidation of a reactant occurs is
called the anode, whereas the electrode where a reduction occurs is called the
cathode. The change in Gibbs free energy of formation for a reaction, Gf ,
determines the equilibrium potential, Ucell, for a given electrochemical cell, consisting
of two electronically connected electrodes immersed in an electrolyte.